The ALMA Reionization Era Bright Emission Line Survey: The molecular gas content of galaxies at z ~ 7

¹ Instituto de Estudios Astrofísicos, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejército 441, Santiago 8370191, Chile
e-mail: manuel.aravenaa@mail.udp.cl
² Cosmic Dawn Center (DAWN), Rådmandsgade 64, 2200 København N, Denmark
³ Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 Copenhagen N, Denmark
⁴ Department of Astronomy, University of Geneva, Chemin Pegasi 51, 1290 Versoix, Switzerland
⁵ Hiroshima Astrophysical Science Center, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan
⁶ National Astronomical Observatory of Japan, 2-21-1, Osawa, Mitaka, Tokyo, Japan
⁷ Leiden Observatory, Leiden University, 2300 RA Leiden, The Netherlands
⁸ International Centre for Radio Astronomy Research, University of Western Australia, 35 Stirling Hwy., Crawley, WA 6009, Australia
⁹ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
¹⁰ Sterrenkundig Observatorium, Ghent University, Krijgslaan 281-S9, 9000 Gent, Belgium
¹¹ Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
¹² Waseda Research Institute for Science and Engineering, Faculty of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan
¹³ Departmento de Astronomia, Universidad de Chile, Casilla 36-D, Santiago 7591245, Chile
¹⁴ Dipartimento di Fisica, Sapienza, Università di Roma, Piazza le Aldo Moro 5, 00185 Roma, Italy
¹⁵ INAF – Osservatorio Astronomico di Roma, via Frascati 33, 00078 Monte Porzio Catone, Roma, Italy
¹⁶ Sapienza School for Advanced Studies, Sapienza Università di Roma, P.le Aldo Moro 2, 00185 Roma, Italy
¹⁷ INFN, Sezione di Roma 1, P.le Aldo Moro 2, 00185 Roma, Italy
¹⁸ Steward Observatory, University of Arizona, 933 N Cherry Ave, Tucson, AZ 85721, USA

Received: 26 June 2023
Accepted: 8 October 2023

Abstract

A key to understanding the formation of the first galaxies is to quantify the content of the molecular gas as the fuel for star formation activity through the epoch of reionization. In this paper, we use the 158µm [C II] fine-structure emission line as a tracer of the molecular gas in the interstellar medium (ISM) in a sample of 𝓏 = 6.5–7.5 galaxies recently unveiled by the Reionization Era Bright Line Emission Survey, REBELS, with the Atacama Large Millimeter/submillimeter Array. We find substantial amounts of molecular gas (~10^10.5 M_⊙), comparable to those found in lower-redshift galaxies for similar stellar masses (~10¹⁰ M_⊙). The REBELS galaxies appear to follow the standard scaling relations of the molecular gas-to-stellar mass ratio (µ_mol) and the gas-depletion timescale (t_dep) with distance to the star-forming main sequence expected from extrapolations of 𝓏 ~ 1–4 observations. We find median values at 𝓏 ~ 7 of μ_mol = 2.6_−1.4^4.1 and t_dep = 0.5_−0.14^+0.26 Gyr, indicating that the baryonic content of these galaxies is dominated by the gas phase and evolves little from 𝓏 ~ 7 to 4. Our measurements of the cosmic density of molecular gas, log(ρ_mol/(M_⊙ Mpc⁻³)) = 6.34_−0.31^+0.34, indicate a steady increase by an order of magnitude from 𝓏 ~ 7 to 4.